"20/20" continues with David Muir. Reporter: It's a hall of shame filled with unlikely faces. Female teachers entrusted to protect, who instead preyed on their male students. Names like Debra Lafave, convicted of abusing a 14-year-old. Mary Kay Letourneau who had sex with her 12-year-old student. She famously told the world their love was real. I felt a very deep love for him. Reporter: And tonight, at the head of that infamous class now, Ms. Murray. 28, married and sleeping with the boy who had just come to America with his new parents and his new dreams. And for that mom who so tried to protect him, it was all crashing down. That text that came to her phone. The teacher thinking she was texting jimmy saying I love you, I miss you, and now her younger son at home was troubled too. Something wasn't right. So your youngest son isn't leaving his room? No. For days? Reporter: And when the 12-year-old did emerge from his bedroom, she couldn't believe what he told her. Fernando came to me and says, "Mom, I have something to tell you." I said, "What is it?" He says, "That smile you have on your face is going to go away when I tell you this." "She was here." I said, "What do you mean, she? Who is she?" He says, "Kathryn Murray was here." And I said, "What?" He says, "Mom, they had sex." Reporter: The little brother had stumbled in on them while looking for the family dog. He says I saw her bra and I saw a couple of condoms on the floor. Reporter: The teacher had been in their own house. This notion that this teacher would come into your home and get into your son's bed with him. Oh, this is sick. Reporter: This was now an outraged mother on a mission. And I say, I need to prove this person was in my house. That's it. She just crossed the line. I put some gloves and I took everything I can take from my son's bedroom, all the covers, pillows, and I called the police. And I say, okay, do the DNA. Let's prove she was here or not. And it was positive. Yeah, DNA from those sheets proved that she was there. Reporter: This is the same teacher you had called the school and said, keep her away from my son. Yes. Reporter: The same teacher they had once thanked for helping their son who spoke so little English. I kept sending e-mails. Thank you for taking care of my son. Reporter: Armed with that DNA the case moves quickly. 15-year-old jimmy and his 12-year-old brother suddenly swept into it and sitting across from investigators. Jimmy determined to protect his teacher. So you've been brought to a room where investigators were asking you what happened. Yes. My brother went first and then me. So I went there and I started lying. I start saying, I just kiss her. Reporter: The investigator said to you they already knew the truth? Yes. And I say, okay. And I start telling the truth. I say, yes, I have relationship with her. And we have sex. Reporter: Ms. Murray is quickly arrested and charged with sexual assault of a child. She's released on bail but ordered to stay away from jimmy, and there is relief at jimmy's house. He's now back at school, captain of the soccer team, and the family is back at the dinner table. Months would go by, he begins to open up to his mother. He started to tell you piece by piece how soon -- Details. Very hard details for a mom to hear. Reporter: He tells her that it wasn't just their house or the hotel. That the sex had all started in that classroom, room 308. And not only does he tell his mom, he tells police, who start scouring that surveillance video looking for evidence. And there it is. Jimmy, leaving Ms. Murray's classroom on the afternoon he says they first had sex. Just six minutes later, watch as Ms. Murray coming out of that same room. The school surveillance showed that they were leaving her classroom on Tuesday, February 7th, at 5:15 P.M. And the school records showed that schools regularly dismissed at 3:30 P.M. Reporter: Ms. Murray is now charged with two more counts of sexual assault of a child and one count of an improper relationship with a student and she is ordered now to where an ankle bracelet. And as the months go by those parents are learning more about how they say Ms. Murray lured their son in. Jimmy's dad reads to me the notes, prosecutors say, the teacher wrote to him. So, this is a poem she wrote to your son? That is correct. Reporter: And what does it say? It says, because of you I feel. I write. I see. I know. I understand. I crave. I desire. I hope. I learn. I love. And because of you I do not hate myself. This is from the teacher-- This is from-- To your son? Reporter: It was back on, jimmy secretly started seeing Ms. Murray. Yes. But when you saw her? Yes. All the feeling that I have for her came back. Reporter: His parents now suspect, even with that ankle bracelet, and facing a possible 20 years in prison, Ms. Murray was still toying with their son's emotions. They hire a private investigator to follow him. And that very night, sitting out front, the investigator sees jimmy leave through a window in the middle of the night. A friend driving him. That investigatoking us back along that fateful route. I was following him on the freeway and I'm thinking, wow, this is really happening. Those parents knew exactly what they were saying. Reporter: They knew this teacher hadn't stopped. I said, okay. Reporter: The private eye suddenly finding herself in front of Ms. Murray's father's house. How quickly do you call Houston police? Once I saw jimmy pull up and go inside, I picked up the phone and called HPD and told them I was working on a case involving Ms. Murray and they recognized the name instantly. Reporter: She calls jimmy's parents, too. As we're driving to Ms. Murray's father's house I'm calling the police. I'm saying, I'm the father. This is the case number. And when we got there, there were seven police cars. Reporter: Now the parents, the private eye, and the police in a dramatic confrontation surrounding the house, guns drawn. The middle school teacher and the 15-year-old inside. They got on the bullhorn and said, miss Kathryn Murray, this is the police--

This transcript has been automatically generated and may not be 100% accurate.

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